It would depend on a type of insulation in a cathedral ceiling section and your climate zone. If you have a properly installed foam type insulation entirely filling rafter spaces above the ceiling; there’s no need (nor way) for ventilating it. If this foam insulation is a closed cell type it would work in warm and cold weather, however, if an open cell foam type insulation has been applied it could result in mold growth during cold season.
Other common types of insulation used between the rafters, like fiberglass, utilize air between the fibers as an insulator. If you’re experiencing cold winters, condensation may develop between the fibers if there’s no ventilation space between the insulation layers sitting on the ceiling’s finishing material (drywall, paneling, etc.) and the roof decking material (plywood, wooden planks, etc.). In order for condensation to evaporate without (very often) causing mold growth is to provide air flow / circulation.
If those rafter bays are sealed you may need to open one of them and check what’s inside. Let me know if you have more questions, just click “comment” below.